Thank you for your question. I phoned the office of Ayatullah Sistani (hA) and explained the situation you have described. Here is their response, assuming that the husband had no other children and his father is not alive:
Inheritance applies to all of the property of the husband (his bank balance, property, car etc), after paying off any debts he had and performing the distribution of his wealth up to one third accordingly if he has made a will for that. It does not apply to things he gifted to others (including his wife) during his lifetime which are the property of those people he has gifted those things to.
The net amount after these deductions is to be split up accordingly:
His mother inherits 1/6
His wife inherits 1/8
His child inherits the rest
Since his child has also passed away, the child's portion is inherited by the wife, which is inheritance from the child.
May you always be successful